The present invention relates to expansion valves for providing flow of liquid refrigerant at a reduced pressure to an evaporator in a refrigeration system having refrigerant circulated from a compressor through a condensor, expansion means and to an evaporator for vaporization therein and return to the compressor. The present invention particularly relates to electrically operated refrigerant expansion valves employed in automotive air conditioning systems where the compressor undergoes rapid and wide variations in speed as a result of changes in engine speed. Systems of this type are also exposed to rapid and wide variations in thermal load on the evaporator and a wide variation of ambient conditions experienced by the condensor.
Heretofore, in designing solenoid operated refrigerant expansion valves for automotive air conditioning systems, problems have been encountered with friction between the armature and the guide means for the armature; and, in some cases failures have been experienced with the armature sticking in a particular position due to friction causing the valve to be thereafter non-responsive to variations in the width of the electrical control signal pulse. Attempts to eliminate such armature sticking by increasing the clearances between the armature and the armature guide have proven impractical where the armature is guided internally and valving action is provided by sliding the internal surface of the armature over cross ports in the internal guide member. Increased clearance between the armature and the cross ports has resulted in undesirable increases in the residual flow of the valve when the armature is in the position for closing the ports; and, has also resulted in noise or buzzing of the armature where the control signal has a frequency on the order of 30-60 Hz.
It has therefore been desired to find a way or means of improving the guidance of the armature in a solenoid operated refrigerant expansion valve in a manner which will maintain the proper residual flow through the valving ports when the armature is in the closed position. It has also been desired to prevent sticking of the armature when moved to any of plural positions for opening the ports in response to a pulse width modulated (PWM) signal. It has been particularly desired to find such a valve construction suitable for use in automotive air conditioning applications wherein the control signal for the valve has a voltage signal level of 12 volts from the onboard vehicle power.